carb and mpg questions????

[csd4682]

I have an 83 d150 with a 318 2wd. I am getting terrible mpg, I'm talking a 30 mile trip is burning up a 1/4 tank of gas. I know these old trucks are not very efficient but it seems worse than it should be. I was wandering what may help, opened air cleaner, exhaust? If i put a opened air cleaner on, will I have to rejet the carb or should I be ok. I have re-jetted a lot of motorcycle carbs but never touched a vehicle carb. any other suggestions would be great as well. Thanks.

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[crazzywolfie]

is it a 2 or 4 barrel? the 4 barrel would get worse gas milage.

[Rudebaker]

Originally Posted by crazzywolfie

is it a 2 or 4 barrel? the 4 barrel would get worse gas milage.

Actually when properly adjusted if you keep your foot out of the secondaries a 4 Bbl. can get better mileage than a 2 bbl. since the primaries on most 4 Bbl. carbs are smaller than the venturies on a 2 Bbl.
On vacuum secondary carbs the opening point can be adjusted so they rarely open.

Prior to adjustments made Monday my '86 D150 was getting about 7.5 MPG. It is a 318, 3 Spd. AT with 2.94 gears. The engine is basically stock with 9,000 miles on an overhaul. It has free flowing exhaust, a Holley 600, 14 X 4 open element air cleaner and has an old 1980's Edelbrock SP2P intake manifold which was suppose to help fuel economy.
I had one on a 350 Chevy and I don't think they really do much. I reset my timing, made sure the vacuum advance was functioning correctly and adjusted the idle mixture on the carb. It was all screwed up, idle vacuum was only 14 inches, after adjustment I got it up to 20 which is much better. I also aired my tires up to 35 PSI. I'm anxious to see what effect this has on mileage.

600 Holley carbs are not the greatest mileage carbs, but are what most people are familiar and comfortable with. One of the best there is is the QuadraJet. It is a very sophisticated carb, anybody that calls them QuadraJunks or QuadraBogs shouldn't be allowed to open their own hood without adult supervision. They require patience to tune properly but once dialed in they are the next best thing to EFI. I've been a fan of the Q-Jet for 25 years now.

Check your exhaust for blockage, older mufflers can collapse inside restricting exhaust flow. Make sure all your tires are fully inflated, check timing, idle mixture, idle speed, etc. Make sure your choke is open completely once the engine is warmed up. Advance your timing as much as you can without incurring detonation under a load. Check your plugs to see if the engine is running rich or lean. Too lean can hurt gas mileage as much as too rich, it can cut power at cruise RPM and you'll be in the throttle more just to maintain speed or to pull long grades and hills. I've had cars where going up a size or two on the primary jets actually improved mileage. An open element air cleaner can help during warm weather but in colder weather it can hurt because it will lean out the engine until it warms up and you'll be pumping the accelerator just to keep the thing running. If you do get one save the stocker for cold weather driving. Another thing to look at is your thermostat, engines will get their best mileage when run at about 195*. Cold thermostats are great for performance but don't do a thing for fuel economy.

Install a vacuum gauge and drive the truck so you maintain the highest possible vacuum at all times. If you can only have one tuning instrument in your tool box it should be a vacuum gauge. They can tell you almost everything you need to know about what is going on inside an engine.

Good luck and keep us posted, I'll post updates on my "mileage hunt".

[crazzywolfie]

Originally Posted by Rudebaker

Actually when properly adjusted if you keep your foot out of the secondaries a 4 Bbl. can get better mileage than a 2 bbl. since the primaries on most 4 Bbl. carbs are smaller than the venturies on a 2 Bbl.

most people don't relise this and have a heavy foot when there truck is starting to feel gutless.

Originally Posted by Rudebaker

I'll post updates on my "mileage hunt".

have you ever tried playing with hydrogen? all the systems i have built leak.

[Rudebaker]

Originally Posted by crazzywolfie

most people don't relise this and have a heavy foot when there truck is starting to feel gutless.

That's where the vacuum gauge can help, when you see that needle drop everytime you stomp on it and start relating that to fuel consumption you start easing up. You need to put the gauge right on top of the dash where you can't help but see it. Adjusting the secondaries to come in later will help to. I've put about 25 miles on mine since I posted this AM and the difference out on the highway is amazing. I was able to pull a steep grade near here with hardly any throttle movement whereas a couple days ago I was in the 4 Bbl. on that same hill just to hold 55 MPH.

Originally Posted by crazzywolfie

have you ever tried playing with hydrogen? all the systems i have built leak.

No, I've read up on it quite a bit but there doesn't appear to be enough payback in it for the trouble. If I do it will be on something else. This is just a parts chaser for other projects.

The problem with most older full size pickups is aerodynamics, they are tall and flat fronted and are trying to punch a pretty big hole in the air. That's one roadblock to MPG's that's pretty hard to overcome.

[csd4682]

Truck has a stock 2 barrel. how can I tell if I have a "vacuum secondary" carb. I know its a holly. I took it apart and cleaned it when I bought the truck. The truck is an 83 and only has 60k on it, but it sat in a field for a few years without moving. been a fun project, but I am really hoping for a little better mpg

[Rudebaker]

"Secondaries" are the back two, or secondary barrels on a 4 Bbl. carb, a 2 bbl. only has "primaries". You mention that it sat in a field for a few years, that's pretty hard on them. Things get stuck, have you checked to make sure the brakes aren't dragging? Make sure all the moving parts, wheel bearings, suspension components, etc. have been well lubricated.

If your carb is in good shape the factory jetting is probably pretty close, open element air cleaners don't have the affect on jetting on cars and trucks as they do on motorcycles, it will lean the mixture slightly but that is what you want up to a point.

A 4 Bbl. can help mileage IF you keep your foot out of it. Depending on how much you want to spend I'd recommend an Edelbrock Performer manifold and carburetor, the 500 cfm carb with electric choke would work well. I have one on my '55 Studebaker, only problem with it is if the engine isn't started every couple days the bowls go dry. Something about the way they are vented allows the fuel to evaporate I'm told. They don't all do it but I have read posts on several other forums from people with the same complaint. It only takes a few seconds for the bowls to fill up again but it can be annoying. My choice would be a QuadraJet, buy a good remanufactured one from Summit or JET if you go that route. Parts store rebuilds can be a real hodge podge of parts. I've used the Summit reman'd. Q-Jet and it worked great. If you go with a Q-Jet get Doug Roe's book on Rochester Carbs, it has very detailed tuning information. There's a newer QuadraJet book out that's supposed to be really good too but I don't recall the title.